# Why do prescription glasses that darken in sunlight fail to darken when the light travels through a car windshield?

I was going to buy this add on, as I almost exclusively use my sunglasses for driving. I'm just stacking them over my normal ones now... but it's quite annoying that I almost paid the extra \$ for this feature. Found out at the last second, luckily.

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Reaction sunglasses (also known as photochromic glasses) go dark in reaction to ultraviolet light with wavelengths from 280-320 nm. The smallest wavelength of visible light is around 390 nm. Ordinary glass blocks over 90% of light with wavelength below 300 nm but allows longer wavelength UV to pass. The glass used in car windows is often treated to block more UV light. This is to protect the interior of the car from light induced ageing but it also helps drivers avoid sunburn. This means that most of the UV light that causes the glasses to react is filtered out by the car windows, but not all.

Another important factor is that the most intense UV light that falls on the glasses when you stand outside is coming directly from the Sun at a high angle, not from the direction you are looking in. In a car this is usually blocked by the roof of the car or a sun visor. UV light is mostly absorbed by any surface it falls on so the ambient UV light inside a car would be quite low even if the windows were open

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After reading through the wikipedia entry on the subject I find your explanation both concise and more accurate as you actually reference wavelengths (rather pertinent to a discussion like this). Thanks much. I will hold off on buying this tech when next I replace my glasses as well, and hope that an alternative or complementary technology is developed in the next decade or so. –  NOTjust -- user4304 Jan 1 '12 at 21:07
Recently got 2 new pairs of glasses (replaced 2011 pair), one with 'transition' coating. They do seem to work at about 20-30% of their maximum inside non-luxury auto-glass from 2010/11 (replacement windshield). The amount of ambient light (or UV reflection maybe) also has an effect. Certainly they 'transition' much more quickly and significantly with direct, unfiltered sun-light. I do not have huge coverage on the lenses though, so medically speaking they do not protect my eyes as well as larger lenses or my non-prescription sunglasses, since UV light will still hit me from the sides. FYI. –  NOTjust -- user4304 Apr 6 '13 at 17:57

These type of glass/plastic responds to UV light. Car windshield has UV filters so the glasses does not darken as much inside cars.

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If I'm not mistaken, this is why when Richard Feynman was viewing the Trinity A-bomb test, he doffed the superdark goggles the army issued and chose to view the explosion through the windshield of a nearby truck. The windscreen filtered the damaging UV light. –  Warrick Oct 11 '11 at 7:19
@Warrick This seems to be a fairy tale in my opinion. An atomic explosion is like looking into the sun directly, dangerous because of very strong visible and IR radiation. You need dark glasses, as You need them when watching a solar eclipse. –  Georg Oct 11 '11 at 8:33
@Georg I recall Feynman giving this anecdote himself in his letters. I listened to the audiobook of his letters and I'm pretty sure he describes it there. –  Warrick Oct 11 '11 at 10:18
Hmmmm, this brings down my esteem for Feynman a bit! Maybe he was tired and looked for the seat in the truck? –  Georg Oct 11 '11 at 10:21
@Georg: Feynman is known for his braggadocio as much as his physics. The story could very well be something he'd tell, exaggerated for comedic effect. –  Jerry Schirmer Oct 11 '11 at 13:00

These glasses probably respond to the amount of ambient light. In a car the ambient light is substantially less than outside. Your eyes are focused on the wind shield, but glasses are also affected by the dark dashboard, ceiling, etc

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Too bad people downvote without commenting. I still believe my answer touches on an important topic. If the car wouldn't have a windshield, but just a hole, the glasses would still hardly respond. Philip Gibbs also refers to this as "another important factor". –  Kris Van Bael Jul 9 '13 at 21:08